Magnetic resonance systems acquire data using strong magnets for providing large static magnetic fields. Gradient coils are provided to "focus" the magnetic fields. The gradient coils and the large static magnetic fields are used to magnetically align nuclei in a desired plane of the sample being imaged or spectrographically studied. A radio frequency (Rf) pulse is used to nutate the nuclei. When the Rf pulse terminates the nutated nuclei precess and return to the aligned condition. As the nuclei precess and return to the aligned state, the movement of the nuclei in the magnetic field generate what are known as free induction decay (FID) signals. It is the FID signals that are most popularly used for imaging and spectrographic purposes.
Special Rf coils or probes are used for transmitting Rf pulses or receiving the Rf FID signals. These probes are energized in the transmitting state with an Rf pulse having a frequency known as the Larmor frequency, which as is well known, is a function of the particular element and the magnetic field strength. The Larmor frequency is also the precessional angular frequency and the frequency of the FID signals.
While any type of magnet can be used to generate the large static field, in a preferred embodiment, a superconducting magnet is used. The specimen or patient is placed within the bore of the superconducting magnet for uniform exposure to the large static magnetic field. The Rf probes are either body probes wherein the entire body of the patient can be within the probe or probes designed to be juxtaposed to particular portions of the body such as the limbs or the head.
The probes must be capable of: resonating at the desired RF frequency; generating homogeneous magnetic fields, and not adding excessive noise to the pulses transmitted or the signals received.
Nothwithstanding the relative efficiency of the proximate probes, the signal to noise ratio (SNR) of the acquired data remains critical because of the very small amplitudes of the FID signals. The SNR decreases because among other things, of unbalances due to stray capacitances in the probes themselves and because of variations in the impedances of the probes when loaded by the patient or sample.
Different patients have different body impedances and therefore load the Rf probes differently. Also the human body, for example, or the specimens themselves in spectroscopic work are not generally considered as being symmetrical bodies. Thus, loading is non-symmetrical loading, and results in variations in the signals received from the probes. Also the probes are variable, among other ways, in that the impedance thereof change with temperature and relative humidity among things.
Another problem encountered in the use of the prior art Rf probes and especially with the larger body probes is that they have relatively low self-resonant frequencies which limit the top frequency that can be used as the Larmor frequency. It has been found that with the stronger fields, such as the 2 Telsa fields, for example, better resolution is obtained in the image. However, since increasing the magnetic strength of the field, increases the Larmor frequency, the self resonant frequencies of the probes tend to be a limiting factor in the magnetic strength that can be used.
Another serious problem, faced by the scientists and designers of MR systems is that the Rf power transmitted by the probes may cause heating of the samples or sections in patients being examined. That is because only a very small portion of the Rf power nutates the nuclei while most of the power causes eddy and dielectric currents in the tissues of the subject which generate heat. This is a "microwave oven" effect. As a matter of fact, the Federal Drug Administration (FDA) in the United States has set a limit on the specific power absorption rate (SAR) of the Rf signal that can be used in imaging humans. The set limit is 0.4 watts per kilogram. Thus, there is a limit on the power that can be used in the Rf probes as a function of the patient's weight. This limit is designed to safeguard the patient from undergoing microwave caused heating damage to tissues.
Most of the probes used in the past were often saddle shaped coils and were linearly polarized. That is the magnetic fields provided by the probes were normal to one of the planes defined by two of the orthogonal axis of the magnet of the MR system. Generally speaking the MR systems are viewed as being XYZ orthogonal systems with the large static magnetic fields assumed to be in the Z direction and the Rf fields assumed to be perpendicular to the XZ plane. The nutated nuclei or spins precess around the Z axis for example, and the effective projection or linear polarization is in the XY plane, while the spins are processing. In the past, due to the linear polarization by the applied Rf pulses, one-half of the generated magnetic lines did not pass through the subject. Accordingly half of the Rf power was not effectively used to nutate the spins.
Still another problem is that the presently available Rf probes cause what are known as radio frequency penetration artifacts to appear mainly on the body images as shaded areas. The artifacts result from standing waves of the RF radiation passing through the tissue at high frequencies which distort the uniformity of the applied radio frequency magnetic field. To attempt to overcome this problem the prior art has implemented an excitation mode wherein the polarization is circular rather than linear. (See an article entitled "An Efficient Highly Homogeneous RF Coil for Whole Body Imaging at 1.5T" by C. E. Hayes et al in the Journal of Magnetic Resonance 163.622-628 (1985)). This mode is also sometimes referred to as a "quadrature mode". The circular polarization in addition to improving image quality reduces the power required to achieve a given nutation of the spins. Accordingly smaller Rf power amplifiers can be used. Also less energy is absorbed by the patient thereby obviating the problem of possibly exceeding the 0.4 watts per kilogram SAR. The sensitivity of the receiver coils to the FID signals are also greater with circular polarization by an amount that increases the signal to noise ratio by a factor of the square root of 2. The circular polarization decreases the Rf power necessary by a factor of 2.
A problem with the circular or quadrature mode equipment has been that the homogeneity of the generated field within the center of the subject does not match the homogeneity of the fields generated by the saddle coils. A related problem with quadrature mode equipment has been in providing coils that can generate the circularly polarized Rf fields without being unduly influenced by loading of the probe due to the patient within coil. The quadrature mode generating equipment is in general unduly influenced by the cross-coupling between the multiple coils that are used to generate the circular polarization.
The prior art attempts at accomplishing circular polarization or quadrature excitation has been accomplished using what is commonly referred to as "bird cage" resonators. The bird cage resonator comprises a pair of spaced apart circular end loops surrounding the patient or the portion of the patient being imaged. The circular spaced apart loops are joined by a plurality of straight conductors or rods defining the length of the probe. One version of the bird cage resonantor has tuning capacitors along the length of each of the rods and matching capacitor along one of the rods. The power input is attached across the matching capacitor. This is known as a low pass bird cage resonator.
Another version of the bird cage resonantor uses rods without the capacitors between the oppositely disposed circular end loops and instead breaks up the circular end loops with capacitors between the rods. This version is known as the high pass bird cage resonator.
As previously mentioned a major problem with bird cage resonator body probes is that they are very suspectible to adverse loading by the body. They repeatedly work fine as long as the subject is a phantom that is completely symmetrical. When a patient is the subject within the bird cage resonator then the inherent lack of symmetry unequally loads the probe and causes artifacts in the images, decreases the signal to noise ratio and requires greater power.
A prior art attempt at combining saddle coils and quadrature polarization is described in an article entitled "Quadrature Detection in the Labratory Frame", by D. I. Hoult et al published in Magnetic Resonance in Medicine, Vol. 1, pp. 339-351 (1984). The article describes a plurality of overlapping saddle coils, which theoretically can be used in quadrature. Implementation of the theory has proven difficult.
Accordingly scientist in the field are still seeking efficient probes for use in MR systems which provide circular polarization.